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REC-xml-names-19990114
Copyright © 1999 W3C (MIT, INRIA, Keio ), All Rights Reserved. W3C liability, trademark, document use and software licensing rules apply. Status of this document This document has been reviewed by W3C Members and other interested parties and has been endorsed by the Director as a W3C Recommendation. It is a stable document and may be used as reference material or cited as a normative reference from another document. W3C's role in making the Recommendation is to draw attention to the specification and to promote its widespread deployment. This enhances the functionality and interoperability of the Web. The list of known errors in this specification is available at http://www.w3.org/XML/xml-names-19990114-errata. Please report errors in this document to xml-names-editor@w3.org. Abstract XML namespaces provide a simple method for qualifying element and attribute names used in Extensible Markup Language documents by associating them with namespaces identified by URI references. Appendices We envision applications of Extensible Markup Language (XML) where a single XML document may contain elements and attributes (here referred to as a "markup vocabulary") that are defined for and used by multiple software modules. One motivation for this is modularity; if such a markup vocabulary exists which is well-understood and for which there is useful software available, it is better to re-use this markup rather than re-invent it. Such documents, containing multiple markup vocabularies, pose problems of recognition and collision. Software modules need to be able to recognize the tags and attributes which they are designed to process, even in the face of "collisions" occurring when markup intended for some other software package uses the same element type or attribute name. These considerations require that document constructs should have universal names, whose scope extends beyond their containing document. This specification describes a mechanism, XML namespaces, which accomplishes this. [Definition:] An XML namespace is a collection of names, identified by a URI reference [RFC2396], which are used in XML documents as element types and attribute names. XML namespaces differ from the "namespaces" conventionally used in computing disciplines in that the XML version has internal structure and is not, mathematically speaking, a set. These issues are discussed in "A. The Internal Structure of XML Namespaces". [Definition:] URI references which identify namespaces are considered identical when they are exactly the same character-for-character. Note that URI references which are not identical in this sense may in fact be functionally equivalent. Examples include URI references which differ only in case, or which are in external entities which have different effective base URIs. Names from XML namespaces may appear as qualified names, which contain a single colon, separating the name into a namespace prefix and a local part. The prefix, which is mapped to a URI reference, selects a namespace. The combination of the universally managed URI namespace and the document's own namespace produces identifiers that are universally unique. Mechanisms are provided for prefix scoping and defaulting. URI references can contain characters not allowed in names, so cannot be used directly as namespace prefixes. Therefore, the namespace prefix serves as a proxy for a URI reference. An attribute-based syntax described below is used to declare the association of the namespace prefix with a URI reference; software which supports this namespace proposal must recognize and act on these declarations and prefixes. 1.1 A Note on Notation and Usage Note that many of the nonterminals in the productions in this specification are defined not here but in the XML specification [XML]. When nonterminals defined here have the same names as nonterminals defined in the XML specification, the productions here in all cases match a subset of the strings matched by the corresponding ones there. In this document's productions, the Note that all Internet domain names used in examples,
with the exception of [Definition:] A namespace is
declared using a family of reserved attributes. Such an attribute's
name must either be
[Definition:] The attribute's value, a URI reference, is the namespace name identifying the namespace. The namespace name, to serve its intended purpose, should have the characteristics of uniqueness and persistence. It is not a goal that it be directly usable for retrieval of a schema (if any exists). An example of a syntax that is designed with these goals in mind is that for Uniform Resource Names [RFC2141]. However, it should be noted that ordinary URLs can be managed in such a way as to achieve these same goals. [Definition:] If the attribute name
matches [Definition:] If the attribute
name matches An example namespace declaration, which associates the
namespace prefix
Namespace Constraint: Leading "XML" [Definition:] In XML documents
conforming to this specification, some names (constructs corresponding to
the nonterminal
The Note that the prefix functions only as a placeholder for a namespace name. Applications should use the namespace name, not the prefix, in constructing names whose scope extends beyond the containing document. In XML documents conforming to this specification, element types are given as qualified names, as follows:
An example of a qualified name serving as an element type:
Attributes are either namespace declarations or their names are given as qualified names:
An example of a qualified name serving as an attribute name:
Namespace Constraint: Prefix Declared This constraint may lead to operational difficulties in the case where the namespace declaration attribute is provided, not directly in the XML document entity, but via a default attribute declared in an external entity. Such declarations may not be read by software which is based on a non-validating XML processor. Many XML applications, presumably including namespace-sensitive ones, fail to require validating processors. For correct operation with such applications, namespace declarations must be provided either directly or via default attributes declared in the internal subset of the DTD. Element names and attribute types are also given as qualified names when they appear in declarations in the DTD:
5. Applying Namespaces to Elements and Attributes The namespace declaration is considered to apply to the
element where it is specified and to all elements within the content of
that element, unless overridden by another namespace declaration with the
same
Multiple namespace prefixes can be declared as attributes of a single element, as shown in this example:
A default namespace is considered to apply to the element where it is declared (if that element has no namespace prefix), and to all elements with no prefix within the content of that element. If the URI reference in a default namespace declaration is empty, then unprefixed elements in the scope of the declaration are not considered to be in any namespace. Note that default namespaces do not apply directly to attributes.
A larger example of namespace scoping:
The default namespace can be set to the empty string. This has the same effect, within the scope of the declaration, of there being no default namespace.
In XML documents conforming to this specification, no tag may contain two attributes which:
For example, each of the
However, each of the following is legal, the second because the default namespace does not apply to attribute names:
In XML documents which conform to this specification,
element types and attribute names must match the production for An XML document conforms to this specification if all
other tokens in the document which are required, for XML conformance, to
match the XML production for The effect of conformance is that in such a document:
Strictly speaking, attribute values declared to be of
types Appendices A. The Internal Structure of XML Namespaces (Non-Normative) A.1 The Insufficiency of the Traditional Namespace In the computing disciplines, the term "namespace" conventionally refers to a set of names, i.e. a collection containing no duplicates. However, treating the names used in XML markup as such a namespace would greatly impair their usefulness. The primary use of such names in XML documents is to enable identification of logical structures in documents by software modules such as query processors, stylesheet-driven rendering engines, and schema-driven validators. Consider the following example:
In this example, there are three occurrences of the name
Another problematic area comes from the use of "global" attributes, as illustrated by this example, a fragment of an XML document which is to be displayed using a CSS stylesheet:
In this case, the XML 1.0 does not provide a built-in way to declare
"global" attributes; items such as the HTML In order to support the goal of making both qualified and unqualified names useful in meeting their intended purpose, we identify the names appearing in an XML namespace as belonging to one of several disjoint traditional (i.e. set-structured) namespaces, called namespace partitions. The partitions are:
In XML documents conforming to this specification, the names of all qualified (prefixed) attributes are assigned to the global attribute partition, and the names of all unqualified attributes are assigned to the appropriate per-element-type partition. A.3 Expanded Element Types and Attribute Names For convenience in specifying rules and in making comparisons, we define an expanded form, expressed here in XML element syntax, for each element type and attribute name in an XML document. [Definition:] An expanded
element type is expressed as an empty XML element of type
[Definition:] An expanded
attribute name is expressed as an empty XML element of type
Slight variations on the examples given above will illustrate the working of expanded element types and attribute names. The following two fragments are each followed by a table showing the expansion of the names:
The names would expand as follows:
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